For people who live in areas that experience harsh winter climates, growing flowers, herbs, fruits and vegetables year-round is very difficult. In those environments, greenhouses are effective. A greenhouse is a structure, usually enclosed by glass or plastic (glazing), which helps to maintain an adequate temperature range throughout the year and a measure of control over the environment for growing healthy plants.
Generally speaking there are two types of greenhouse design; conventional and passive solar. Both types of greenhouses serve the same function, which is to provide an internal environment conducive to plant growth. There are, however, important differences in design and operation to consider.
Conventional greenhouses are designed to harness the sun's energy in the form of light for plant growth. Typically, conventional greenhouses are constructed of transparent, uninsulated glass or plastic. Ideally, these designs allow light in during the day for photosynthesis and heat. Overheating frequently occurs and is a problem with these designs. Furthermore, the poor insulative quality of the glass and plastics often leads to a detrimental loss of heat at night requiring supplemental heat sources, such as CO2 producing fossil fuels like natural gas or propane. This requires large amounts of energy, especially in colder regions, and is a major on-going expense that decreases the profit margin of the grower, or is passed on to the consumer through higher product cost. Many conventional greenhouses are oriented with the axis in a north-south direction, which does not optimize use of the low angle winter sun, which stays close to the southern horizon during the coldest time of the year.
The term passive solar greenhouse generally refers to greenhouses designed to harness the sun's energy to use for both light and heat during cold winter months. The three basic elements of passive solar greenhouses are; an efficient collection of solar energy, the storage of solar energy as heat, and the prevention of heat loss during and following collection periods. It is these characteristics that separate passive solar greenhouses from conventional greenhouses.
Plants use the energy in sunlight to convert CO2 (from the air) and water (from the soil) into sugars—a process called photosynthesis. The sugars are used to metabolize and to build new stems and leaves and when plants burn their sugars for food. CO2 is produced as a waste product. Since photosynthesis is powered by sunlight, plants absorb more CO2 than they give off during the daytime, but at night, when photosynthesis is dormant, the opposite occurs.
A greenhouse uses a few basic scientific principles to maintain its interior microclimate. Sunlight is the primary source of heat for a passive solar greenhouse. Sunlight passes through transparent materials, such as glass or clear plastic and when it hits an opaque, or less transparent surface, some of that light is transformed into heat. The darker the surface, the more light gets transformed into heat.
In most traditional greenhouses comprised of glass or plastic, there is almost always some heat loss which requires additional heat sources to maintain acceptable temperatures during colder periods. The goal is to trap energy inside the greenhouse to comfortably heat the plants, the ground, and soil inside it. Ideally, the air near the ground is warmed and prevented from rising and dissipating too quickly. In order for plants to flourish, a greenhouse must provide the appropriate amounts of light humidity, and warmth and the key to operating an effective greenhouse is being able to maintain a reasonably stable climate to support plants.
Air movement also has a large impact on the morphology, physiology, and reproduction of the plants as it affects the temperature of the leaf, gas exchanges and resistance of the boundary layer and, therefore, photosynthesis, transpiration and water use. Limited air movement, for example, hinders the supply of CO2 to the stomata of the leaves for photosynthesis. Therefore, it is necessary to achieve a minimum horizontal air movement for CO2 supply to the leaf stomata. Increases in CO2 levels generate an increase in photosynthesis and subsequent increase in yield as well as induces an improvement in the water use efficiency. The recommended CO2 concentration depends on the species and variety, as well as the environmental conditions. For vegetables, it has been recommended not to exceed 1500 ppm for cucumber or 1000 ppm for tomato and pepper. Recently, 1000 ppm has been considered a suitable maximum limit for all species except cucumber, aubergine, and gerbera. An excess in CO2 in tomato plants may cause abnormally short leaves or the rolling of the leaves, whereas in other crops it may cause leaf chlorosis. Ventilation is the most economic method to limit CO2 depletion in the greenhouse air; however, in most contemporary systems it only allows the maximum to reach ambient conditions (350-400 ppm).
During the day (photosynthesis), plants will generally use twice as much CO2 than they give off from cellular metabolism. After the photosynthetic period, plants will continue respiring and give off CO2 without using any, so that there can be a buildup above ambient levels overnight in a well-insulated greenhouse for the start of photosynthesis the next morning.
Thus, there is a delicate balance between maintaining proper CO2 concentrations and temperatures within the greenhouse. To date, the most effective way to maintain control over the internal greenhouse CO2 concentrations has been via the application of pure CO2 combustion gasses, enrichment with small burners, or enrichment from a central boiler. However, these methods are expensive and wasteful.
Light regulation is practiced in a greenhouse for reasons including altering the length of daylight hours (increasing or reducing them) and increasing photosynthesis (complementing the naturally available light and/or extending the length of the day with artificial light. The objective is to maximize photosynthesis by maximizing the light interception by the greenhouse. Standard techniques employed in greenhouse design have been via supplementary artificial lighting techniques, such as lamps, etc. But again, such techniques require an energy source, such as electricity and are thus expensive to use and maintain.
As mentioned above, plant cultivation in climates that experience temperatures below the plant's ideal growth temperature range relies on a controllable microclimate often provided by greenhouses. When these greenhouses experience interior temperatures that fall outside the plant's ideal growth temperature range, the majority of these greenhouses rely on either 1) active heating methods from fossil fuels or electricity driven heaters to raise the interior temperature or 2) active ventilation from electric-driven fans to exhaust heat to lower the interior temperature to maintain the correct temperature range. Additionally (as discussed above), these greenhouses may use electric grow lights to increase photosynthetic light available to these plants. These greenhouses frequently experience carbon dioxide levels below the outside (ambient) levels during the photosynthetic period and often require carbon dioxide enrichment equipment, to optimize plant productivity during the photosynthetic period.
Passive solar greenhouses have been recognized as offering tremendous promise for growing winter crops in areas in the northern hemisphere. For example, the increasing popularity of the “Chinese Solar Greenhouse” has greatly reduced energy demand and carbon dioxide emissions throughout China and elsewhere. Gao L-H, Qu M, Ren H-Z, Sui X-L, Chen Q-Y, and Zhang Z-X. Structure, function, application, and ecological benefit of a single-slope, energy-efficient solar greenhouse in China. HorTechnology, June 2010 20(3) 626-631. However, the authors also cite problems associated with the Chinese solar greenhouse design including severe CO2 depression, high humidity levels, low light intensity, overheating in the spring and fall, and the need for much better thermal mass. Other standard greenhouse problems include phototropism (e.g., growth or movement of a sessile organism toward or away from a source of light), winter heating and protection from extreme weather events.
Problems such as these are addressed by the current system. Through the techniques and construction described herein, the current system is able to affect the critically important levels of temperature, humidity, carbon dioxide, and natural light to maximize plant production in the system, and require only seeds, soil (nutrients), and water as external inputs. Worldwide, conventional greenhouse designs ignore many of these parameters then add energy intensive systems as mentioned above to correct the inherent weaknesses of their designs. The current system uses a unique combination of design features to create a passive system that eliminates or greatly reduces the input of active, or fossil fuel and electric driven systems, to control the greenhouse's interior temperature, light levels, humidity levels and carbon dioxide levels. The design allows for better capture, distribution, storage, and release of solar energy or its thermal energy form for enhanced winter plant growth and greatly reduces the costs associated with heating, grow lights, active ventilation, and carbon dioxide enrichment.